1. Field of the Invention
The present invention relates to a liquid property detecting device.
2. Description of Related Art
A liquid property detecting device detects a concentration of a component included in liquid. For example, an ethanol concentration is detected in a mixed liquid in which gasoline and ethanol are mixed, when the mixed liquid is used as a fuel for a vehicle engine. When ethanol is used for the vehicle engine, exhaust gas emitted from the vehicle engine has less toxic component, compared with a typical engine using only gasoline.
However, a suitable air-fuel ratio is different between the mixed liquid and gasoline. The vehicle engine is required to have the suitable air-fuel ratio so as to provide higher heat efficiency and to reduce the toxic component included in exhaust gas. Therefore, when the mixed liquid is used for the vehicle engine, an amount of ethanol included in the mixed liquid is measured. The measurement of ethanol concentration is important.
JP-A-6-3313 discloses a liquid property detecting device to detect an ethanol concentration in a mixed liquid in which gasoline and ethanol are mixed. The liquid property detecting device has a nonconductive tube and electrodes arranged around an outer face of the nonconductive tube. When liquid to be measured is introduced into the nonconductive tube, the electrodes and liquid located between the electrodes define a capacitor to be a concentration sensor. When charge and discharge are repeated relative to the concentration sensor through a switch, a voltage proportional to the ethanol concentration is output based on a discharge current and a resistance.
However, in this case, when the liquid to be measured is constructed by only gasoline, a leak resistance between the electrodes becomes infinite, because gasoline is not conductive. That is, a dielectric constant between the electrodes is approximately equal to zero. In contrast, when liquid such as ethanol or water is mixed in gasoline, the leak resistance is decreased, and the dielectric constant between the electrodes has a positive value.
The leak resistance may affect detection accuracy of the liquid property detecting device. Therefore, the affecting by the leak resistance is required to be eliminated so as to provide higher detection accuracy.